The disclosure of Japanese Patent Application No. 2000-7871 filed on Jan. 17, 2000 is incorporated herein by reference in its entirety.
1. Field of the Invention
This invention relates to a guiding apparatus for a slidable member driven by a linear motor, more particularly, to a guiding apparatus for a tool or a workpiece supporting unit used in a machine tool or the like.
2. Description of the Related Art
A grinding head guiding apparatus for a grinding machine has been known by International Patent Publication (PCT) No. WO97/44158 (hereinafter xe2x80x9cfirst related artxe2x80x9d). In the first related art, since one linear motor is disposed under the grinding head, magnetic pulling force generated between magnetic coil unit and permanent magnet unit acts on a pair of right and left fixed guiding surfaces downwardly (in a vertical direction). Therefore, both of the magnetic pulling force and weight of the grinding head act on the fixed guiding surfaces. On the other hand, hydrostatic pockets are arranged on a pair of right and left guiding surfaces (hereinafter xe2x80x9cgrinding head guiding surfacesxe2x80x9d) which are arranged underside of the grinding head and are opposed to the pair of right and left fixed guiding surfaces each other. And hydrostatic pressure generated by fluid supplied to the hydrostatic pockets makes the grinding head float against the magnetic pulling force and the weight of the grinding head. In such a construction, only upward surfaces of the fixed surfaces serve as the guiding surface to guide the grinding head in the vertical direction. When the hydrostatic pressure generating on the fixed surfaces for floating the grinding head balances with resultant force of the magnetic pulling force and the weight of the grinding head, a vertical position of the grinding head is kept at a desire position.
Japanese Patent Publication No. H09 (1997)-150334 discloses another guiding apparatus (hereinafter xe2x80x9csecond related artxe2x80x9d). In FIG. 4 of the second related art, two linear motors are disposed between a grinding head and a base so that magnetic pulling force is generated in a vertical direction. And a pair of right and left fixed guiding portions having an upward guiding surface and a downward guiding surface each other is constituted. Meanwhile, the grinding head has a pair of right and left upside surfaces being opposed to the downward guiding surfaces of the fixed guiding portions each other, and a pair of right and left underside surfaces being opposed to the upward guiding surfaces of the fixed guiding portions each other. Hydrostatic pockets are arranged on each of the upside surfaces and the underside surfaces of the grinding head, so that a position of the grinding head in the vertical direction is adjusted by hydrostatic pressure generated by fluid of supplied to the hydrostatic pockets.
In the first related art, because the vertical position of the grinding head is kept to balance the hydrostatic pressure with resultant force of the magnetic pulling force and the weight of the grinding head, fluctuation of the grinding force act on the grinding head in the vertical direction makes a vertical position of the grinding head to be precarious. Especially in a case that a non-circular workpiece such as camshaft is ground by a grinding wheel rotatably supported at a front end of the grinding head, it should be considered that large grinding force generating upward and downward alternatively acts on the grinding head through the grinding wheel. Particularly, resultant force of the magnetic pulling force and the weight of the grinding head should be configured to be extremely large quantity relative to the hydrostatic pressure to obtain high rigidity for keeping vertical position of the grinding head. This causes a mechanical contact of the fixed guiding surfaces with grinding head guiding surface, and a lack smooth movement of the grinding head.
Further, since thrust of the linear motor for moving the grinding head acts on the grinding head above the guiding surfaces, moment causing a pitching motion of the grinding head occurs easily in structure of the guiding apparatus of the first related art.
In the second related art, since the grinding head is moved by guiding upward surfaces and downward surfaces of a pair of right and left horizontal guiding members, the problem that is precariousness of the vertical position of the grinding head is solved. However, since a linear motor is disposed at a lower position far than the horizontal guiding members, thrust of the linear motor for moving the grinding head causes easily a pitching motion of the grinding head.
Furthermore, in the first related art, a bearing portion of rotatably supporting a grinding spindle provides on the grinding wheel is disposed at a front end of the grinding head, which portion is overhung from hydrostatic guiding surfaces formed on the underside surface of the grinding head. That is, any hydrostatic guiding surface is not formed at an underside surface portion of the grinding head corresponding to the bearing portion, and the grinding head is moved under a condition that the bearing portion protrudes from a front end of the hydrostatic guiding portion formed at underside surface of the grinding head. In this structure, it occurs easily such a bad influence that causes fluctuation of the grinding force which acts on the grinding head trough the grinding wheel.
Moreover, in a guiding apparatus for guiding a grinding head of a grinding machine disposed a heavy grinding spindle bearing portion at the front end of the grinding head, there are some technical requirements as follows:
(1) To guide the grinding head on the fixed guiding surfaces by uniform supporting force through a whole of the guiding surfaces including underside of the grinding spindle bearing portion, thereby avoiding to concentrate mass of the grinding head on the hydrostatic guiding surface adjacent to the grinding spindle bearing portion;
(2) To design each of hydrostatic pockets disposed to the underside surfaces of the grinding head along the moving direction of the grinding head as equivalent specification, thereby simplifying a pressure oil supplying apparatus for supplying pressure oil to the pockets;
(3) To facilitate for maintenance to detach the magnetic coil unit of the linear motor attached on the underside surface of the grinding head along longitudinal direction thereof for maintenance.
In view of the foregoing, an object of the present invention is to avoid tilting a slidable member such as a grinding head even if magnetic pulling force of a linear motor acts on a guiding surface, and is to improve rigidity of the guiding surface so as to raise the natural frequency in a pitching direction of the slidable member, whereby high servo gain for controlling the linear motor can be set.
Another object of the present invention is to keep a posture of the slidable member even though force such as machining force acting on the slidable member fluctuates.
The other object of the present invention is to provide a guiding apparatus suited for guiding a grinding head of a grinding machine, which can satisfy at least one of the aforementioned technical requirements.
In order to achieve the above objects, the present invention provides a guiding apparatus for a slidable member driven by a linear motor. The guiding apparatus comprises: a base; a pair of horizontal guide members fixed on the base along a longitudinal direction thereof and guiding a slidable member; a primary consisting a linear motor and attached on an underside surface of the slidable member; and a secondary consisting the linear motor and attached on an upside surface of the base along the longitudinal direction thereof.
According to a first aspect of the present invention, a vertical position of a portion of opposing the primary and the secondary is approximately same as that of a vertical position of the horizontal guide members guiding the slidable member.
Since trust of the linear motor acts on the slidable member at the same vertical position as that of a guiding portion of the horizontal guide members, moment causing a pitching motion of the slidable member does not occur.
According to a second aspect of the present invention, the guiding apparatus further comprises a pair of temporary support members attached on the base of both sides of the secondary along the longitudinal direction thereof, which support the primary so as to avoid the primary sticking to the secondary when the primary is detached from the slidable member.
Since sticking the primary to the secondary is avoided by the temporary support members, the primary is easily detached from the slidable member for maintenance.
According to a third aspect of the present invention, the guiding apparatus comprises a supporting unit attached on a front end of the slidable member, wherein a primary is not attached at an under portion of the supporting unit.
The magnetic pulling force does not act on the slidable member at the under portion attached on the supporting unit on which portion weight of the supporting unit act. Therefore, the slidable member is guided to the horizontal guiding members by uniform supporting force through a whole of the horizontal guide members including the under portion attached on the supporting unit.
According to a forth aspect of the present invention, the slidable member has a pair of ribs extending along the longitudinal direction of the slidable member therein, which define an approximately trapezoid-shaped space disposed at a center of the slidable member in the lateral direction and a pair of triangle-shaped space disposed both sides of the trapezoid-shaped space.
Since the slidable member is consisted trusses formed by the ribs, both of the rigidity and lightweight of the slidable member can be improved.